The volume of air transportation is placing increasing demands on air traffic control systems. Air traffic control systems may utilize surveillance radar systems for detection of aircraft approaching and within a controlled region, beacon systems for activating transponders on aircraft equipped therewith, communications between air traffic controllers and aircraft, wind shear detectors, weather radar, terminal approach systems, terminal approach systems for use with parallel runways, wake vortex monitoring, and possibly other functions. The various equipments required at each airport are individually expensive, their independent siting requires extensive installation and large land area, and also requires extensive communications lines and facilities for interconnection of the equipments with a control center. The independent sites must each be provided with security and maintenance, which increases costs. Present air traffic control primary surveillance radars such as the ASR-9 are mechanically scanned fan-beam systems.
Mechanically scanned reflector antennas for surveillance use generally use a "cosecant squared" fan-beam radiation pattern to provide coverage in elevation while scanning in azimuth. Mechanically scanned systems cannot advantageously be adapted for common use for tracking and either final-approach control or atmospheric-disturbance monitoring, because the reflector antenna has substantial inertia, and cannot be moved quickly from one position to another. In radar, any condition generating a reflection, such as an aircraft or a localized weather phenomenon, is termed a "target". For aircraft final approach control, the delay from one rotation of the reflector antenna to the next is so long that proper aircraft control may not be possible under all circumstances, especially with highspeed targets such as aircraft, and atmospheric disturbance targets may change or move significantly during a rotation. Long pulse repetition intervals (PRI) are required to provide unambiguous coverage over long distances using pulse Doppler waveforms. The long PRI requires the rotating-reflector antenna to dwell for a relatively long time at each incremental azimuth position, so the antenna rotational speed cannot be increased without reducing its maximum unambiguous range. For an instrumented range (maximum range for which the equipment is designed and optimized) of 60 nautical miles (nm), the ASR-9 completes a 360.degree. scan in about 5 seconds. One nautical mile equals 1852 meters or 1.1508 statute miles.
The long-range requirement also requires the use of relatively high transmitted power to reliably detect small targets. High transmitted power implies a relatively higher peak transmitter power or a longer duration transmitter pulse (also known as a "wider" pulse). Assuming a maximum available peak power, longer range implies a longer duration transmitted pulse. A longer duration pulse tends to reduce range resolution, which is the ability to distinguish among targets which are at similar ranges. Pulse compression techniques can be used to improve range resolution in spite of the longer pulse duration, thus eliminating the need for high peak power short pulses, but the minimum range at which a target can be detected increases with the transmitted pulse length. Thus, if the transmitter pulse duration is 100 microseconds (.mu.s), the minimum distance at which a target may be detected is about 8 nautical miles (nm). Clearly, a surveillance radar using pulses of such a duration cannot be used to detect aircraft which are landing or taking off from an airport. An additional problem associated with pulse compression is the appearance of range sidelobes (as distinguished from antenna sidelobes) in addition to the main range lobe. The time position, or range, of the main lobe is the position that is tested for the presence of a target and for estimating the parameters of that target (reflected energy or power, closing speed, fluctuations in echo power and closing speed, etc.). The presence of range sidelobes on the compressed pulse results in interfering echoes which originate at ranges other than the range of the main lobe. This interference, known as "flooding" can cause erroneous estimates of the echo characteristics in the range cell (i.e., range increment) covered by the main lobe. Prior art techniques for suppressing range sidelobes include the "zero-Doppler" technique, in which the range sidelobe suppression scheme is based in part upon the assumption that the interfering echoes, as well as the desired echo, have a closing velocity that has no significant Doppler phase change or shift over the duration of the uncompressed pulse. The Doppler phase shift .phi..sub.DV across the uncompressed pulse is 2.pi. times the product of the Doppler frequency shift and the uncompressed pulse duration (i.e. .phi..sub.DV =2.pi. f.sub.d T.sub.0 radians). When this Doppler phase shift is actually zero or very small, moderate sidelobe suppression is achievable with the zero Doppler design. However, the zero Doppler design is very sensitive to small Doppler frequency shifts, making deep sidelobe suppression impossible for applications in which very deep sidelobe suppression is desired, as in weather mapping, clear air turbulence detection, and microburst detection.
Electronically scanned array antennas are inertialess, and may be capable of rapid scanning. The rapid scanning ability gives rise to the possibility that various air traffic control and atmospheric monitoring uses could be multiplexed with the surveillance. An array antenna using a centralized power transmitter and a "corporate" feed has lossy transmission-line components, including power splitters, between the transmitter and the element of the array antenna. Such losses may make it difficult to achieve the desired power gain with antennas of reasonable size, low-power phase shifters, and moderate-power transmitters.
An active phased-array radar may provide improved reliability over a single-transmitter radar by virtue of its many transmitter modules. Also, it may provide high power gain by virtue of its many transmitter modules, and because power losses occur at low power levels before final amplification, which results in low power losses between the transmitters and their antennas. The active antenna architecture also provides reduced system noise during reception because the majority of the receiver losses follow low-noise amplification. Because of the inertialess scanning, it provides the possibility of integration of functions other than surveillance, thereby providing an overall cost reduction.